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This is the first time I am selling a car in the UK, are there any precautions I should take regarding test drives?
Should I ask a potential buyer if he has an insurance? Or should I notify my insurance that other people will drive my car?

Any potential buyer driving your car with your permission under their own insurance will be covered by third party insurance on their policy. That means if they have an accident your car will not be covered for any damage caused to it, the insurance will only cover any claims against the driver with regard to damage to another vehicle(s) and/or person(s).
If you advise your insurance that other drivers will be driving your car they will want full details of the drivers same as if you had a named driver on your policy.
I would advise you to accompany any test drives if you are prepared to risk a potential buyer to drive or take them out for a test drive with you driving it.